1. Field of the Invention
The invention pertains to methods for using information obtained about the physiology of a person to make decisions regarding possible treatment of potentially life-threatening heart disorders, more particularly those disorders of the ventricular arrhythmatic type known to be associated with lethal sustained ventricular tachycardia.
2. Brief Description of the Prior Art
Cardiac arrhythmias are potentially life-threatening irregular heart rhythms usually occurring in the context of a diseased or damaged heart. The most serious cardiac arrhythmias are those originating within the large, lower heart chambers known as the ventricles. In a large proportion of patients who have ventricular arrhythmias, the risk of future sudden death is evaluated by performing a procedure known as an electrophysiologic study (EPS) in which the physician attempts to electrically "induce" an underlying rhythm disturbance known as sustained ventricular tachycardia (SVT). Ventricular tachycardia (VT) is a condition in which the ventricles, or lower chambers, of the heart experience a series of rapid abnormal contractions. If VT can be induced, the patient undergoes appropriate treatment to reduce his future risk of death. If no arrhythmia can be induced, such a risk is acceptably low and no further therapy is recommended.
One ventricular arrhythmia which is commonly seen is known as nonsustained ventricular tachycardia (i.e., ventricular tachycardia which lasts for only short periods) or NSVT. Since a significant number of patients who have NSVT go on to develop the more serious SVT, electrophysiologic testing is usually recommended for them. However, not all persons who have NSVT are subject to inducible arrhythmias and thus derive no benefit by undergoing the electrophysiologic testing.
Because electrophysiologic testing is potentially dangerous and requires expensive hospitalization for periods up to 3-4 weeks, it would be extremely useful to have a physiologic indicator which would help the physician determine which persons would benefit most from EP testing and would decrease the number of patients unnecessarily subjected to EP testing. Such an indicator would also give both the physician and patient the confidence that additional testing is appropriate when indicated.
In some patients with NSVT, a beat-to-beat variation in cycle length (where cycle length is defined as the time interval, measured in milliseconds (msec), between the R wave peaks of successive QRS wave complexes) can be observed to occur. In others, successive cycle lengths appear quite uniform. The degree of variation of cycle lengths during an episode of NSVT is referred to as cycle length variability. Examples of two electrocardiograms which exhibit variability in cycle length are shown in FIG. 1A and FIG. 1B.
Previous work by the inventor revealed that cycle length variability in episodes of NSVT is significantly lower in persons with inducible VT than in persons who experience NSVT but in whom VT is not inducible. K. P. Moulton, et al., "Cycle Length Variability in Nonsustained Ventricular Tachycardia: A Marker for Inducible Sustained Ventricular Tachycardia," PACE, 13:508 (1990). However, this work did not reveal a specific way that a cycle length variability value (hereinafter referred to as CLV) could be used to clearly delineate whether an individual exhibiting episodes of NSVT would be likely to be "inducible" and thus gain benefit from the extensive process of electrophysiologic testing.